Pressure or force transducers have been constructed in many different types. Generally, force transducers are pressure transducers, but not all pressure transducers are force transducers. Thus, the more general type is a pressure transducer, and is so designated herein. In utilizing a semiconductor such as silicon, three types of pressure transducers are currently in use: piezojunction, piezoresistive, and capacitive. Piezojunction transducers use the change in beta or the reverse junction I.sub.cbo to measure the change in pressure. These are highly sensitive, but extremely unstable, and therefore have seen little commercial development. Piezoresistive devices are currently in wide use and many devices are available. They use the stress-induced changes in silicon resistivity and tend to be reasonably stable, but are not particularly sensitive, i.e., about 10.sup.-5 volts/volt/torr is the theoretical limit, with about 5.times.10.sup.-4 volts/volt/torr being the practical limit. Capacitive pressure transducers can be more sensitive by more than an order of magnitude and, if properly applied, are more stable than either piezo effect device type. Such semiconductor capacitive transducers are illustrated in U.S. Pat. Nos. 4,420,790 and 4,415,948.
Discrete capacitive pressure transducers currently enjoy wide acceptance as quality pressure transducers, and have even been applied toward the miniaturization of these pressure transducers. U.S. Pat. Nos. 3,397,278 and 4,415,948 also show a way of electrostatically bonding a silicon wafer to a glass plate in the forming of such pressure transducers. All pressure transducers, regardless of type, currently have analog outputs and utilize outboard analog-to-digital converters of varying types, if the device is to be coupled to a digital system. There is a type of pure digital pressure transducer presently available based on quartz technology. The disadvantages of the system are that one has to use quartz crystals in a bucking mode and they are very expensive systems.
The usual practical approach is the measurement of the changed capacitance based upon an AC parameter, i.e., the capacitance of a resonant tank circuit or the capacitance used in a divider or bridge configuration. The tank circuits are not widely used, due partly to the difficulties with precise control of the inductance. The change in capacitive impedance type has a variety of measurement techniques, with the simplest and most prone-to-error being the single-ended method. Differential and bridge measurement techniques are also used. In all of these methods, an AC driving waveform is utilized with DC resolution.
Another system currently used is a timed constant current charge and discharge cycle. This is utilized by primarily the dedicated digital capacitance meters, but is not applicable, due to distributed capacitance, to the small values of change of capacitance used in capacitance pressure transducers, e.g., 10.sup.-12 farads.
It has been suggested in some of the prior patents to utilize some digital processing of the signal, including a variable duty cycle, such as in U.S. Pat. Nos. 4,272,822; 4,227,419; and 4,398,426. Also, it has been suggested to utilize an astable multivibrator of the non-balanced type, as shown in U.S. Pat. Nos. 4,392,383 and 4,295,376.